In order to cope with the insufficiency of frequency bands available for radio communication, cognitive radio has been studied and developed in recent years. The basic concept of cognitive radio is that a radio terminal is configured to have the function of recognizing or knowing the radio environment around it and to select a frequency and scheme to be used in radio communication in accordance with the radio environment, whereby the spectral efficiency is enhanced. Specifically, the cognitive radio terminal scans frequency bands to recognize the radio environment thereby detecting the use state, and use an available frequency band. This enables radio communication with good spectral efficiency utilizing all the frequency bands, theoretically.
Patent Document 1: Japanese Patent Application Laid-Open No. 2007-300419
Patent Document 2: Japanese Patent Application Laid-Open No. 2000-207679